Presently, testing for defects for quality control on test objects such as wire, rods or tubes is frequently performed by using eddy current (EC) technologies. The testing often involves having the test objects travel along a work path, passing through eddy current probe(s).
The technical issues associated with conventional eddy current NDT/NDI systems include a) maximizing test coverage of test object surfaces, b) accommodating test objects of different sizes by means of efficient probe adjustment, and c) maintaining a uniform distance between the sensing surface of the probes and the test object surface (i.e. the lift-off distance).
Drawbacks associated with these systems are as follows.
U.S. Pat. Nos. 3,919,628 and 4,641,092 present a well practiced inspection technique that employs a single eddy current probe affixed to a rotating head that typically provides coverage helicoidally around a test object at high rotational speeds. This “rotating head” technique is characterized by a high rotation speed and a relatively complicated mechanical design that requires considerable maintenance. The helical gap between rotations typically limits the longitudinal resolution making the detection of very small longitudinal defects troublesome. Furthermore, it is not feasible to inspect test objects that are not round shaped with this technique.
An alternate inspection technique employs one or multiple eddy current coils (elements) with each coil completely encircling the test object. The size and shape of the “encircling coils” must substantially match the size and shape of the test object due to the critical lift-off requirement for the coils. Various attempts have been made to overcome this limitation, such as those presented in U.S. Pat. No. 5,412,319 and U.S. Pat. No. 5,638,000. However, various disadvantages involving the encircling coil technique remain. The most notable of these is the dependency of detection sensitivity on the circumference of the object being inspected resulting in significantly limited detectability of circumferential defects. Consequently, it is not possible to localize the circumferential position of defects. Also, with this type of arrangement, although it is possible to detect large defects, substantially smaller defects remain undetected because of the signal averaging process used in this type of system.
To overcome the limitations associated with encircling coils and the rotating head technique, an alternate EC inspection technique employs many smaller coils that are aligned in an array surrounding the outer surface of a test object being moved through the inspection zone of the EC array. U.S. Pat. No. 4,785,243 describes an example of this technique. Using this method, surface coverage and detectability are improved compared to encircling coils and rotating head techniques. However, these techniques require a probe that exactly matches the size and shape of the test object in order to limit the lift-off because many eddy current elements are disposed in a rigid fixture. One unique probe, which is relatively expensive, must be used for each test object size and shape. In addition, operation cost associated with changing the probes to accommodate various test objects is quite high.
Another solution to facilitate insertion of the test object into the inspection zone and to accommodate for slight variations of test object size involves the use of multiple small coils affixed to multiple probe heads. The surface of the probe heads onto which the eddy current elements are affixed is shaped to match the surface of the test object. These probe heads can be brought into proximity of the surface of the test object by various means. U.S. Pat. No. 4,101,832 describes one such approach for which a plurality of eddy current receiver coils are mounted on and supported by a plurality of probe heads that surround the part, each of the four probe heads covering a respective quadrant of the circumference of the test object. Although some flexibility in accepting test object with variable sizes is provided, this flexibility is very limited, particularity for small diameter test object.
Other inspection systems provide a somewhat limited means of lift-off compensation. These systems provide gain versus test object diameter graphs for each element for a given calibration diameter, typically fixed. As these gain versus test object diameter graphs are provided at the factory, changing the diameter of the calibration test object in the field is problematic and leads to relatively extensive recertification to obtain relevant gain versus inspection test object diameter graphs for the new diameter.
It would therefore be beneficial to provide an eddy current probe assembly that is easier and more efficient for the inspection of one test object size to another while maintaining high test resolution and high inspection productivity. It is therefore more economical to build such an EC system with fewer kinds EC probe heads and less down-time associated with changing probe heads.